fix: override Content-Length header in auth proxy config

The proxy was forwarding the original Content-Length header to the
proxied service, which caused issues with request body processing.
Now setting it to an empty string to prevent nginx from passing the
original content-length value.

docs/deploy.md

@@ -42,7 +42,7 @@ location = /_auth_proxy {
     internal;
 
     proxy_pass_request_body on;
-    proxy_set_header Content-Length $content_length;
+    proxy_set_header Content-Length "";
     proxy_set_header X-Real-IP $remote_addr;
     proxy_set_header X-Original-Host $host;
     proxy_set_header X-Original-URI $request_uri;